1. Field of the Invention
The present invention relates to an optical signal processor including a diffraction grating device.
2. Related Background Art
As an optical signal processor including a diffraction grating device, one disclosed in Japanese Patent Application Laid-Open No. 2001-4447 has been known for example. The optical signal processor disclosed in the publication intends to demultiplex light with an excellent wavelength resolution by causing a diffraction grating device to diffract the light twice. By propagating light in the direction opposite from that at the time of demultiplexing, the optical signal processor can also multiplex the inputted light having a certain wavelength region and output thus multiplexed light.
Meanwhile, the state of polarization of light varies as characteristics of the optical transmission line fluctuate upon changes in environments and the like, whereby the light to be fed into and processed by the optical signal processor does not always have a constant state of polarization. Therefore, it is desirable for the processing of light in the optical signal processor to be less dependent on the state of polarization of input light. However, the efficiency of light diffraction in diffraction grating devices has been known to vary depending on the polarization direction of incident light.
For overcoming such a problem, the optical signal processor disclosed in the above-mentioned publication disposes a half-wave plate on the optical path between the first and second diffracting actions caused by the diffraction grating device. The half-wave plate has an optic axis in a direction parallel to a plane perpendicular to the optical axis of incident light. When linearly polarized light having a polarization direction inclined by an angle θ with respect to the optic axis is incident on the half-wave plate, the polarization direction of the light is rotated by an angle 2θ, and the light is emitted as linearly polarized light having a polarization direction inclined by an angle (−θ) with respect to the optic axis. If the angle θ is 45 degrees, the half-wave plate can output the incident linearly polarized light as linearly polarized light having a direction orthogonal thereto. By utilizing such an effect of the half-wave plate, the optical signal processor disclosed in the above-mentioned publication rotates the polarization direction of light by 90 degrees between the first and second diffracting actions caused by the diffraction grating device, thereby lowering the dependence on polarization.